An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. Often that RF signal exhibits frequencies in a predetermined frequency band that is licensed by a regulatory agency for a particular use. The RF power amplifier boosts the power of this RF communication signal to a level sufficient so that the signal, when it propagates to an antenna, will be broadcast in such a manner that it will meet the communication goals of the RF transmitter.
Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. In other words, the RF communication signal conveys both amplitude and phase information, and the RF power amplifier should faithfully reproduce both the amplitude and phase content of the RF signal presented at its input. While perfect linearity is a goal for any linear RF power amplifier, all linear RF power amplifiers invariably fail to meet it. The degree to which the goal of perfect linearity is missed leads to unwanted intermodulation, nonlinearities, and spectral regrowth.
The regulatory agencies that license RF spectrum for use by RF transmitters define spectral masks with which transmitters should comply. The spectral masks set forth how much RF energy may be transmitted from the RF transmitters in specified frequency bands. As transmitter technology has advanced, and as increasing demands have been placed on the scarce resource of the RF spectrum by the public, the spectral masks have become increasingly strict. In other words, very little energy outside of an assigned frequency band is permitted to be transmitted from an RF transmitter. The trend is for further out-of-band transmission restrictions in the future. Accordingly, unless the spectral regrowth that results from any nonlinearity in the amplification performed by an RF power amplifier is held to a very low level, the RF transmitter will be in violation of its regulatory spectral mask.
In conventional RF power amplifiers, the linearity parameter is counterbalanced against power-added efficiency. Power-added efficiency, hereinafter referred to simply as “efficiency”, is the ratio of the RF output power to the sum of the input RF power and the applied direct-current (DC) power. In conventional RF transmitters, improvements in efficiency have been achieved at the expense of linearity.
Amplifiers are often classified into various classes, depending upon how they are operated and upon a conduction angle parameter. The conduction angle is the portion of a complete RF signal cycle over which an amplifier operates or conducts. Class A operation corresponds to amplifier conduction over an entire RF cycle (i.e., a 360° conduction angle); class B operation corresponds to amplifier conduction over only half of an entire RF cycle (i.e., a 180° conduction angle); class AB operation corresponds to amplifier conduction between class A operation and class B operation (i.e., 180°-360° conduction angle); and, classes C-F all have conduction angles less than 180°. Classes A, AB, and B are all considered suitable for linear amplification applications but are less efficient than classes C-F. The higher classes (e.g., C-F) are deemed to be nonlinear classes and are more efficient, often much more efficient. Each linear class is less efficient than all nonlinear classes. Class A is both the most linear class of operation and the least Efficient.
A need exists to achieve RF power amplifier linearity consistent with strict, modern regulatory spectral masks, but at the same time improve efficiency. One application where this need is felt is in connection with cellular handsets. A cellular handset is a battery-operated device. So, improved efficiency translates directly into either longer battery charge-retention times, or the use of smaller batteries and the provision of smaller cell phones. But cellular handsets transmit signals at relatively low power levels (typically less than 1 W peak) and over a relatively narrow bandwidth (typically less than 5 MHz). This low power and low bandwidth application affords the opportunity to trade a small amount of linearity degradation for significant efficiency improvements.
Another application that needs RF power amplifier linearity consistent with strict, modern regulatory spectral masks and at same time as much efficiency as possible is a cellular basestation. A significant percentage of the overall lifecycle costs of a typical cellular basestation is dedicated to purchasing electrical power, and the RF power amplifier is one of the largest power consumers in the cellular basestation. As up-front costs for placing cellular base stations in service diminish, this on-going power cost is expected to become a larger portion of the overall lifecycle costs.
Cellular basestations tend to be high power RF transmitter applications (e.g., greater than 5 W and often much greater). In general, a cellular basestation should be capable of transmitting at a power level roughly equal to the sum of the power levels at which a maximum number of cellular mobile stations that can communicate with it transmit. The number of mobile stations active at any instant can vary widely, placing a wide dynamic range over the transmission power requirements of the basestation.
Moreover, it has become popular to combine the signals from several different channels within a cellular basestation's RF transmitter to form a multichannel signal having a wide bandwidth (e.g., greater than 10 MHz). While a compatible mobile station need transmit in only one of the channels at any instant, the basestation will transmit in multiple channels simultaneously, and its RF power amplifier should linearly amplify over a wide bandwidth that encompasses all of the channels. The use of multichannel signals also causes a peak-to-average power ratio (PAPR) of such signals to increase to extreme levels. In other words, rarely occurring signal peaks can be at far greater amplitudes than the average signal amplitude. And, these rarely occurring extreme peaks can appear and diminish at a rapid rate compatible with the wide bandwidth.
A variety of RF power amplifier efficiency enhancements related to variably biased RF power amplifiers have been proposed, at least for low power, narrow bandwidth applications. Biasing relates to the typical DC voltages and currents that are applied to power inputs and signal inputs of amplifiers so that they will reproduce an input signal in a desired manner. Using Lateral Diffusion Metal Oxide Semiconductor (LDMOS) field-effect transistor (FET) terminology, the biasing refers to typically DC voltages applied to the drain and gate of an LDMOS, FET, RF power amplifier. For conventional variably biased RF power amplifiers, these bias voltages are modulated to achieve improved efficiency with the goal of harming linearity as little as possible.
With the envelope-elimination and restoration (EER) technique, also known as the Kahn technique, the amplitude component of a communication signal is separated from the phase component. Then, the phase component is amplified in a highly efficient amplifier configured for a nonlinear class of operation. The amplitude component is restored by varying the bias voltage at the power input (e.g., the drain) of the nonlinear class amplifier commensurate with the amplitude component of the communication signal. In a narrowband, low power application, the EER technique achieves significant efficiency enhancement over the linear classes of operation. But a significant price is typically paid in linearity. The EER technique is not used in high power and wide bandwidth applications because, rather than realizing efficiency enhancement, efficiency deterioration is the likely result along with reduced linearity. Efficiency deterioration would result from attempting to generate a high power bias voltage that exhibits a bandwidth consistent with the amplitude content of a wide bandwidth signal.
Another variably biased RF power amplifier technique is the envelope-following technique. Envelope following differs from the EER technique in that both the amplitude and phase components of the communication signal are amplified in a linear-class amplifier. But like the EER technique, power input bias voltage is varied in a manner commensurate with the amplitude content of the communication signal. Accordingly, bias voltage need not be greater than it needs to be to accommodate the RF signal being amplified in a linear class of operation on an instant-by-instant basis. Efficiency enhancements result when compared to traditional linear-class amplifier operation using static DC biasing voltages. Typically, timing issues are less critical than in the EER technique, and the linearity deterioration is less severe than in the EER technique as a result. But a linearity penalty still results, and the envelope-following technique is not used in high power and wide bandwidth applications because, rather than realizing efficiency enhancement, efficiency deterioration is the likely result.
Another variably biased RF power amplifier technique is the envelope-tracking technique. Envelope tracking differs from the envelope-following technique in that the envelope of the RF communication signal is not followed completely. This lowers the switching frequency requirements in the power supply that generates the bias voltage applied to the RF power amplifier's power input, resulting in some efficiency gain to offset an efficiency loss suffered by not completely following the envelope. And, timing issues become less critical, at least in narrow bandwidth applications, so that linearity need not suffer greatly. But a linearity penalty still results, and nothing is provided to ensure that the linearity penalty does not result in the violation of a spectral mask.
These efficiency enhancements have had little success in connection with a high power, wide bandwidth application, such as in a cellular basestation. On the other hand, while a cellular basestation's RF transmitter should be able to linearly amplify even the rare extreme peak amplitudes, the fact that these peaks occur so rarely affords an opportunity to benefit from efficiency enhancements because it is the average power level that influences power costs.
With modern strict spectral masks, what is needed is an efficiency enhancement that does not simply trade-off linearity for power efficiency but that actively manages linearity and efficiency together so that as much efficiency as practical can be achieved without violating the spectral mask. And, the efficiency enhancement should be usable even in a high power, wide bandwidth application.